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Bob in Pennsylvania modifies his outdoor boiler

Bob installed his outdoor boiler in late 2000. This is his update.

This is Bob's second report on his experiences with his outdoor boiler. You'll find his first report here.

I reported my experiences with my outdoor wood boiler to the woodheat.org site in late 2000. The webmaster said, "We hope he'll send us an update as he learns more." So here it is.

I managed to get through the Winter of 2000-2001 with my wood boiler, but was not at all happy with the smoke and the 22% net delivered efficiency. During the Fall of 2001, I decided to void my warranty in the interest of science and made three basic modifications to the unit. First was to line the combustion chamber with a refractory material. I decided on "regular" firebrick for the bottom, back and sides, and alumina fiberboards for the top baffles. I also made a secondary combustion chamber by adding a false roof of the same alumina fiberboard panels, supported by the firebrick sides. I did not add a means for providing air directly to the secondary chamber, figuring I would just increase the draft to give excess air in the primary chamber, which would then carry through to the secondary chamber to oxidize the smoke and other unburned gases. Finally, I added a chimney top draft inducer with a variable speed motor control, so that I could adjust the chimney draft anywhere from .02 inch to .15 inch W.C.

More bricks line the sides and rear and the top baffle was added. The baffles are made from alumina fiberboard sheets.The improvement was amazing. On the first trial from a cold start, I had a net delivered efficiency of 42%, this even with a lot of heat taken up by the firebrick mass. As in previous tests, I let the system cool down by operating the house circulators until the water jacket temperature was back down to 120 degrees F. Since the firebrick was now warm, the second trial was even better! I got at least 48% efficiency. I say "at least" 48%, because at that point, the water temperature had reached the set point on the aquastat, shutting down the draft inducer and closing the intake damper.

Here is the false ceiling installed. The exhaust leaves the firebox at the back of the false ceiling, flows forward and does a 180° turn around the leading edge of the top baffle and goes to the back of the boiler and out to the chimney. Most of the heat transfer happens in this last run to the exit.Another improvement was the delightful lack of smoke! To be sure, while starting up, the boiler did smoke, as does any wood fire, but once all the wood had "caught", it was possible to adjust the draft so that there was no visible smoke. I found that it was best to start with the draft at maximum until the chimney stopped smoking. This would usually take 20 minutes from a cold start, 5 to 10 minutes after adding wood to a hot bed of coals. After that, the best results were obtained by backing off the draft to the point where I just started to get smoke and turn it up a bit. The best efficiency was obtained by adjusting the draft as the fire progressed, turning it down to minimum when the wood was near the ember phase of the burn. If I didn't feel like going out every half hour and fiddling with the draft, the best compromise draft setting seemed to be .05" W.C. once the fire was going.

The front baffle encloses the secondary combustion chamber where the flue gases do the 180° turn.I did have some problems with the high heat cracking the original calcium silicate panels, on the third trial, the false firebox ceiling and baffles collapsed into the firebox. I had to wait half a day for everything inside to cool down enough to safely handle. I rebuilt the inside using alumina fiberboard panels and they have stood up for three seasons now.

These modifications reduced the amount of the wood that can be handled in one load to about one half of original, but because of the improved efficiency, the boiler will still go for about 6 hours before reloading as before. On above freezing days, I have to give it 3/4 loads, else it will cycle off and I'll lose all the efficiency in a smoldering fire. I also found out that very small loads (less than half) don't work well, as the temperature never gets hot enough to light off the smoke and gases. One way to avoid this problem would be to add a heat storage means, such as insulated water tanks in the basement. That way the unit could even run in the spring and fall at maximum efficiency and little smoke.

The system is known to contain 150 gallons of water. Water weighs 8 pounds per gallon, for a water weight of 1200 pounds. The BTU is defined as the amount of heat required to raise one pound of water 1 degree F. The btu/hr rate was determined by timing how long it took to raise the water temperature by one degree, under the test conditions stated below. The boiler comes equipped with a digital thermometer with a stated accuracy of 3%

The refractory bricks and panels run so hot the carbon burns off.The net delivered efficiency was determined by loading the boiler with a known weight of wood, typically 40 pounds. The wood for each trial was red oak, cut from the same tree, seasoned and stored in the same manner. Before each trial, ashes and coals from previous fires were removed from the boiler. The heat from the system was removed by allowing the house circulators to operate until the boiler temperature was down to 120 degrees F. At this time, the house circulators were turned off and the valves to the house heating system closed. The power to the oil burner was also switched off. The circulator for the wood burner was left running, so as to prevent stratification of the water inside the wood boiler.

Heat in the firebox was so much more intense that the draft damper solenoid on the outside of the door failed. Ceramic fiber board shields were added to the door to protect it. This method also inserted the transmission loss through the piping system into the measurements, since the water circulated from the wood boiler, to the house through the inactive oil boiler and back to the wood boiler. There may have been some extra losses through the inactive oil boiler, but these are believed to be negligible, as the boiler was well insulated and the measured stack temperature of the oil boiler while inactive was the same as the ambient temperature. The wood was then ignited and allowed to burn until the boiler temperature no longer showed an increase. At that time the temperature was recorded. The embers and unburned wood, if any were weighed. The heat output was determined as detailed in the previous paragraph. The input was determined by multiplying the weight of the wood consumed by 6000 (an average value stated for seasoned red oak). Efficiency was determined by dividing the BTU output by this input value.